Cardiovascular and metabolic diseases are among the leading causes of mortality and morbidity in the US. There are also substantial disparities in cardiometabolic health depending upon racial/ethnic background and sex that can be traced back to adolescence, the developmental period when youth begin to choose their own social environments and start to make decisions that impact their current and future health. While adolescent physical activity (PA), screen time (ST), and sleep duration are known modifiable behavioral determinants of cardiometabolic health, pubertal timing can also shape an adolescent's activity behaviors, directly through their own behavioral choices, but also indirectly through the behavioral choices of their friends. Therefore, a reduction in disparities in cardiometabolic risk between population subgroups may be achieved by identifying the early social, biological, and behavioral pathways that shape later-life health. The National Longitudinal Study of Adolescent to Adult Health, the largest, most comprehensive longitudinal survey of adolescents ever undertaken, is an ideal dataset to test hypotheses about how adolescent social relationships and biological factors may interact to shape health as youth transition into adulthood. In the proposed research, we will use state-of-the-art statistical social network models that allow us to evaluate how the simultaneously changing structure of social relationships, changing friend activity behaviors, and pubertal timing within one's friendship group jointly shape adolescent activity behaviors as well as later-life cardiometabolic risk. An especially innovative aspect of this research is that we can use objectively-measured biomarker data in young adulthood to study the interaction of adolescent social network, biological, and behavioral precursors on later cardiometabolic health. Young adulthood represents an age when chronic illnesses begin to emerge, and when pre-disease pathways (indicated by objectively-measured hypertension, high sensitive C-reactive protein, cholesterol, glycosylated hemoglobin, and overweight/obesity) can be assessed to gain insight into the mechanisms by which social experiences influence future disease onset. With rich information on the contextual factors that shape human development and health, we can thus evaluate hypotheses regarding the joint importance of pubertal maturation, activity trajectories, and friends' pubertal maturation and activity trajectories on these markers of young adult cardiometabolic health. This proposal is responsive to PA-14-176 as research to identify: (1) social and biological mechanisms that promote positive sustainable health behaviors in children and youth, (2) contextual influences on lifelong health behaviors, and (3) sources of racial/ethnic and gender disparities in risk. Results from these secondary analyses in a nationally representative sample of 132 schools will allow us to adjudicate competing explanations for how adolescent social conditions are linked to adult health, identifying optimal network targets for a future intervention to reduce health disparities before the onset of chronic disease.